1. Field of the Invention
This invention relates to a method for the production of a semiconductor laser device which emits laser light from an end facet thereof, and more particularly, it relates to an improved method for the production of such a semiconductor laser device which can attain high reliability even when operated at a high output power level for a long period of time.
2. Description of the Prior Art
A semiconductor laser device which emits laser light from an end facet thereof is a typical example of the semiconductor devices produced by use of the cleavage of semiconductor crystals. A semiconductor laser device of this type has a Fabry-Perot resonator having a pair of semiconductor facets and functioning on the basis of the difference in refractive index between the semiconductor crystals and the air outside the device.
In recent years, semiconductor laser devices such as described above have widely come into practical use as light sources for optical disc driving units and the like. When semiconductor laser devices are used as the light sources for write-once optical disc driving units or rewritable optical disc driving units, they are required to have high reliability even at a high output power level of about 40 to 50 mW. Furthermore, for the purpose of attaining higher operational speed of an entire system including an optical disc driving unit, there is a demand for semiconductor laser devices which can attain laser oscillation at a still higher output power level. When semiconductor laser devices are used as the light sources for high-resolution laser printers or for optical pumping of solid state laser devices such as a YAG laser, they are required to attain laser oscillation at an output power level of 100 mW or more.
The high output power operation of such a semiconductor laser device, however, causes the deterioration of its end facet from which laser light is emitted. The deterioration in the light-emitting facet increases the current required for driving the semiconductor laser device, and eventually it becomes impossible for the laser device to attain laser oscillation. Therefore, with respect to semiconductor laser devices, it is difficult to attain high reliability at a high output power level.
The principal cause for the deterioration of the light-emitting facet is now described. First, heat is generated locally at the light-emitting facet due to the high optical density at this facet and also due to non-radiative recombination caused by the surface state. As the temperature in the area near the facet increases, the band gap in that area becomes smaller, which in turn increases the absorption of light. The increase in the light absorption generates carriers, which are then trapped in the surface state, and non-radiative recombination of the carriers occurs. This further generates heat in the area near the light-emitting facet. This process is repeated until the temperature in the area near the facet reaches the melting point of the semiconductor, resulting in facet breakdown.
For the prevention of such deterioration in the light-emitting facet, a semiconductor layer having a large band gap, i.e., a large-band-gap layer, may be formed on the facet. For example, it has been devised to form a semiconductor layer having a larger band gap than the energy of laser light emitted (see Japanese Patent Publication No. 55-27474).
The inventors have proposed the formation of a graded-band-gap layer on at least one of the cleavage planes of semiconductor crystals, which function as a pair of resonator facets (see Japanese patent application No. 1-60422, which corresponds to U.S. patent application Ser. No. 07/489,180). The graded-band-gap layer has a band gap which increases gradually with an increase in the distance from the cleavage plane. Thus, the carriers generated in the vicinity of the light-emitting facet are drawn strongly into the inside of semiconductor crystals due to the drift caused by the grading of the band gap, as well as the migration usually caused by the diffusion. This greatly reduces the probability that the carriers will be trapped in the surface state near the light-emitting facet. Furthermore, because the band gap of the graded-band-gap layer is larger than that of the laser oscillating region including the active layer, the absorption of light in the vicinity of the light-emitting facet is reduced. As a result, facet deterioration can be prevented, allowing the semiconductor laser device to attain stable oscillation at a high output power level.
FIGS. 6a to 6c show a conventional process for producing a semiconductor laser device of the above-mentioned type having a large-band-gap layer (e.g., graded-band-gap layer). The following will describe the conventional process for producing a GaAs or GaAlAs semiconductor laser device of this type, which emits laser light from an end facet thereof.
First, as shown in FIG. 6a, on a substrate 111 made of, for example, GaAs, a multi-layered structure 113 including a GaAs or GaAlAs active layer 112 is grown by a known method such as liquid phase epitaxy or vapor phase epitaxy. Then, the wafer thus obtained is cleaved by a known cleavage method so as to obtain a prescribed cavity length, resulting in a plurality of laser bars 115 as shown in FIG. 6b. At this time, cleavage planes 114, which function as a pair of resonator facets, can be obtained.
After cleaving, as shown in FIG. 6c, an SiO.sub.2 film 116 is formed on the planes of the laser bar 115 other than the cleavage planes 114 by plasma chemical vapor deposition or the like. Then, on the cleavage planes 114, a GaAlAs large-band-gap layer (e.g., GaAlAs graded-band-gap layer) is formed by vapor deposition such as molecular beam epitaxy or metal organic chemical vapor deposition. The GaAlAs polycrystals which have been grown on the SiO.sub.2 film 116 are removed by etching, followed by removal of the SiO.sub.2 film 116.
Next, metal electrodes are deposited on the upper face of the multi-layered structure 113 and the lower face of the substrate 111, respectively. Then, a reflecting film of low reflectivity is formed on the light-emitting facet, and a reflecting film of high reflectivity is formed on the other facet. Finally, the laser bar 115 is cleaved to form a plurality of semiconductor laser devices.
In the above-described conventional process, however, the wafer composed of the substrate and the multi-layered structure is first cleaved to form cleavage planes, resulting in a plurality of laser bars. Thereafter, the large-band-gap (e.g., graded-band-gap layer) and the reflecting films are formed on each of the laser bars. Thus, the step of forming these layers and films must be performed for each of the laser bars, so that the production process becomes complicated. The complicated production process makes it difficult to attain a stable production of semiconductor laser devices with high quality. Furthermore, since semiconductor laser devices are obtained by the cleavage of each laser bar, only a small number of the semiconductor laser devices can be obtained by a single cleavage step, which reduces the productivity.